The ECG records fluctuations in body surface electrical potential generated by the sequential depolarization and repolarization of the heart.
Depolarization of the atria generates a low amplitude wave called the P wave.
Once the excitatory impulse has depolarized sufficient ventricular myocardium to generate a measureable electrical field at the body surface, a complex wave form called the QRS is recorded.
Ventricular repolarization begins at the epicardial surface and proceeds inward to the endocardial surface, usually producing a prominent but simple deflection in the ECG called the T wave.